Upgrade breaking dependencies (Supertest 4.0.1 included)

This was blocked because of the following issue in Supertest but now it seems to be OK: ladjs/supertest#557

No issues expected.

package.json

@@ -7,7 +7,7 @@
   "homepage": "https://github.com/kiwicom/fetch",
   "description": "Production ready fetch function with advanced capabilities like retries with delay and request cancellation after timeout.",
   "dependencies": {
-    "@kiwicom/js": "^0.5.0",
+    "@kiwicom/js": "^0.6.0",
     "cross-fetch": "^3.0.0"
   },
   "devDependencies": {